1. Field of the Invention
The invention relates to a weighted absolute difference based noise reduction method and apparatus for adaptively improving image quality.
2. Description of the Related Art
Noise is a disturbance, especially a random and persistent disturbance that obscures or reduces the clarity of a signal. In display and printer technologies, noise degrades quality because it produces artifacts on the displayed or printed image. There are two distinct types of noise, each distinguished by amplitude. The first includes Gaussian noise, Poison noise, uniform noise, and the like. We refer to the first type of noise collectively as Gaussian noise. Gaussian noise amplitude spreads to a full range of gray scale and is inherent in most electronic communication systems. The second type of noise is termed salt and pepper noise because it leaves black or white spots randomly spread on the image. Scratches or dust on recording media and digital transmission errors often cause salt and pepper noise.
An averaging or smoothing filter reduces Gaussian noise by averaging the gray level difference between a target pixel and surrounding pixels. Unfortunately, averaging blurs the image.
A median filter reduces Gaussian and salt and pepper noise without blurring the image. The median filter, however, removes lines that are oriented differently than it and whose width is less than half its size. For example, a three point horizontal median filter will remove vertical and diagonal lines that are one pixel wide. A three point vertical median filter will remove horizontal and diagonal lines that are one pixel wide and so on. The result is perhaps clearer but distorted images.
Accordingly, a need remains for a single process effective at reducing both Gaussian and salt and pepper noise.